A standard flexible coupling has an inner member centered on and rotatable about an axis and formed with a plurality of radially outwardly projecting teeth defining a plurality of radially outwardly open pockets and each having an outer end. An outer member coaxially surrounding the inner member is centered on and rotatable about the axis and formed with a plurality of radially inwardly projecting teeth defining a plurality of radially inwardly open pockets and each having an inner end. The inwardly open pockets each confront a respective one of the outwardly open pockets and respective elastomeric bodies are each engaged in a respective one of the inner pockets and the respective outer pocket to angularly couple the members to each other.
As described in German patent document 2,630,506 of Hagin the outer ends of the inner teeth and the inner ends of the outer teeth can either overlap radially or be spaced from each other radially. When they overlap, as also described in U.S. Pat. No. 2,184,183 of Fykse, the elastomeric coupling bodies are stressed primarily in compression and it is impossible, even if the bodies are destroyed, for the members to rotate freely relative to each other, since in the absence of the coupling bodies the teeth will angularly engage each other. In the system where the outer ends of the inner teeth lie on a surface that is radially inward of the inner ends of the outer teeth, the bodies are stressed in shear and it is possible, if these bodies fail, for the two members to rotate freely relative to each other.
The main disadvantage of the system where the teeth overlap radially is that for a given rotation direction only every other coupling body is actually in use. The alternate bodies are wholly unstressed. When the teeth do not overlap, all the bodies are stressed and in use, regardless of rotation direction, but with this system in the event of overheating or some other situation that causes the bodies to fail, the coupling ceases to transmit torque at all.